This invention relates to a biaxially stretched polyester film excellent in physical properties, particularly in slipping property.
Polyester films prepared from saturated linear polyesters such as polyethylene terephthalate are useful as packaging materials, photographic films, magnetic tapes, etc. owing to their excellent mechanical properties, heat-resistance, weathering resistance, electric insulation and chemical resistance, etc. However, polyester films have a high friction coefficient which affords a bad influence upon processing efficiency in stretching or upon workability in take-up, cutting and coating. Thus, a film which has a low friction coefficient and is excellent in slipping property has been highly demanded.
It is known that the slipping property of a polyester film can be improved by incorporating fine particles therein so as to make fine irregularities at the surfaces of the film. For attaining this purpose, there are known two methods, of which one is the external particle method wherein fine particles inert to a polyester such as titanium dioxide, kaolinite, talc or silica are added to the polyester during its production, and the other is the internal particle method wherein fine particles are formed by the reaction of a carboxylic acid component, an oligomer or a phosphorus compound with a metal compound during the production of the polyester. The internal particle method is superior to the external particle method in the following respects: (i) the equipment investment can be decreased because the apparatuses for pulverization, classification and dispersion of particles are not required; (ii) the use of a dispersing agent, which may deteriorate the heat resistance and electric properties of the resulting polymer yet is essential in the external particle method for preventing the formation of fish eyes by cohesion of particles, is unnecessary; (iii) the particles formed in the internal particle method are generally low in hardness so that the resultant product is excellent in abrasion resistance; (iv) the particles formed in the internal particle method have a good compatibility with a polyester so that the formation of voids on stretching can be avoided; and (v) the refractive index of the formed particles is quite close to that of a polyester, and therefore the transparency of the resultant product is excellent.
In the internal particle method, the particles are usually produced from the residue of a catalyst for ester-exchange such as an alkali metal or an alkaline earth metal, and the amount and size of particles are controlled by a phosphorus compound (e.g. U.S. Pat. No. 4,096,109). However, this conventional procedure is not satisfactory in that it has the following disadvantages: (i) the particles of large size are apt to be produced, and the resulting product is not only inferior in transparency but also contains fish eyes; (ii) the scales are frequently produced in the polymerization reactor, peel off into the polyester product and form fish eyes; (iii) the polymerization conditions must be strictly controlled in order to keep the amount and size of particles constant; (iv) the particle concentration tends to lower under the conditions for minimization of the particle size, and the uniform formation of fine particles in a high concentration is difficult; and (v) the concentration and average size of particles can be controlled only within a narrow range which can not meet all the requirements of users.
In recent years, the requirements of users to the surface properties of films have diversified and the base films having various surface properties are demanded depending upon usages. For example, in case of the films for magnetic tapes, the smoothness of surface is strictly required in one usage, while the slipping property is strictly demanded and the smoothness may be sacrificed to some extent in another usage. Further, the requirements to surface properties are remarkably different according to users even in the same usage. Since the surface properties depend on particle concentration, particle size distribution and kind of particle materials, it is necessary to establish a technique which enables a satisfactory control of the particle concentration, particle diameter and particle size distribution in the manufacturing process of polyester to satisfy the users' demands. However, such a technique has not been established until now.